1. Field of the Invention
The present invention relates to a method of forming a colored band of graded intensity having a desired transverse pattern of graded intensity along a longitudinally extending edge portion of a transparent substrate of glass or plastic. If the substrate is a glass sheet, the colored band of graded intensity is transferred to a plastic sheet used as an interlayer in a laminated glass-plastic safety window during the lamination of said window to form a band having a desired intensity pattern including a smooth boundary between the coated and uncoated portions and free of any disturbing pattern.
Laminated glass has been provided with a band of color along an upper edge when installed in a windshield to protect the occupants of a vehicle from overhead light. It is especially desired that the inner edge of the color band decrease gradually in intensity from an upper portion of essentially uniform maximum intensity toward a lower portion of the windshield having a maximum light transmission for the windshield.
While this invention is especially concerned with the partial coating of glass or plastic sheeting including interlayer material that is a component of laminated glass used in automobiles in such a manner that it is unnecessary to differentially stretch a partly coated plastic interlayer sheet to provide a curved cut-off line for the edge of the colored portion, it is also applicable in the partial coating of substrates used for other purposes, such as other windows in vehicles and in buildings.
The present tendency in automobile design involves increasing the proportional area of glass windows with an object of improving visibility. As the total area of the glass installed in the automobile increases, there is a corresponding increase in the amount of light transmitted to the interior of the automobile. Unfortunately, in sunshine this gives rise to a rapid temperature increase within the vehicle with consequent discomfort to the occupant. The side windows can normally be retracted into the frame of the automobile to provide some relief from the discomfort. However, the windshield and the backlight are usually fixed in position.
It has become common practice to modify the transmission properties of the windshield and backlight so as to reduce the amount of infrared radiation that passes into the vehicle. In case of a laminated windshield, one technique has been to tint the interlayer partially and laminate the partially tinted interlayer with glass sheets. The backlight is usually not laminated but is made of a single sheet of tempered glass. Tinting a backlight has been accomplished by applying a coating to one of the surfaces of the glass. The most suitable coatings have been metal oxide coatings produced by pyrolysis of a metal salt composition that is sprayed onto a heated surface of a glass sheet when the latter is at elevated temperature. The metal salt of the composition applied to the hot glass hydrolyzes into a metal oxide film upon contacting the heated glass. Such coatings have high infrared absorption combined with good transmission properties in the visible portion of the spectrum. Typical metal oxide films are disclosed in U.S. Pat. No. 3,021,227 to Richardson, U.S. Pat. No. 3,244,547 to Orr et al and U.S. Pat. No. 3,078,693 to Lytle. These coatings are also used on glass sheets forming part of a laminated glass-plastic window.
Each of the prior art techniques has some drawbacks. When laminated safety glass windows are fabricated using a tinted plastic as an interlayer, it is necessary to purchase dyed plastic and provide separate storage facilities for dyed plastic and for undyed plastic. Furthermore, the cost of partially dyed plastic obtained from a supplier is very expensive, necessitating a high price for the windshield containing such tinted interlayer sheets as a component thereof. In addition, partially dyed plastic must be stretched differentially to provide a curved boundary between the coated and uncoated portions prior to lamination to curved glass. Controlling the curvature of the boundary represents a further cost of making the ultimate product, particularly when the curve is controlled by stretching.
It is difficult to obtain metal oxide coatings whose optical properties formed by applying a metal salt composition conform to a desired pattern of light transmission because of the difficulty of maintaining uniform consistency of the coating composition, a uniform temperature pattern in the substrate or from substrate to substrate and uniform solubility in the metal salt composition, among other controls that are needed for coatings having suitably controlled optical properties. Generally, metal oxide coatings produced by pyrolysis of metal salt compositions have a mottled effect due to the difficulty in atomizing the spray composition into droplets of uniform composition from droplet to droplet. Also, the thickness required for metal oxide films having the desired reduced transmission characteristics is in the range where iridescent patterns are readily observable. Prior to the present invention, it has been virtually impossible to produce coatings having a main portion of uniform maximum intensity and a portion of controlled width of graded intensity by the aforesaid method.
Films having low coefficients of light transmission have been produced on transparent substrates such as glass by cathode sputtering. U.S. Pat. Nos. 3,506,556 and 3,655,545 to Gillery and Pressau disclose a typical method of forming such films. Unfortunately, cathode sputtered films are expensive to produce and the need exists for a more economical process.
The concept of applying paints and other tinting compositions by an electrostatic spray system has been developed. U.S. Pat. No. 3,645,477 to Cowen discloses an air atomized electrostatic spray device in which air is supplied to the device for the purpose of atomizing a liquid to be sprayed. The air is also employed for operating a self-contained electrogasdynamic power generator for charging the atomized coating material and for maintaining an electrostatic depositing field having one terminus adjacent the locus of atomization. A highly charged atomized mist is imparted to a substrate to be coated. Electrostatic spraying develops a coating of a given intensity characterized by a low transmission coefficient more rapidly than older spraying methods.
The mist so produced comprises highly charged particles that mutually repel one another during the electrostatic spray process. This mutual repulsion causes the particles to spread over a wide area en route to a substrate to be coated. Hence, the electrostatic spray process is suitable for coating an entire substrate uniformly. When an electrostatic spray is applied to an exposed portion only of a substrate, the mutual repulsion of the spray particles causes the electrostatic spray particles to deposit on a large area outside of the exposed portion to which the electrostatic spray is applied and form a pattern of gradually reducing intensity within said large area outside of said exposed portion. Even the interposition of a mask or shield, which controls the boundary in operations in which hydrolyzable salt compositions and sprays other than electrostatic sprays are applied to form coatings in the form of a band, fails to avoid extensive areas of fade-out in coatings applied by electrostatic spraying.
Since the details of the electrostatic spray device does not form part of the present invention and such devices are readily available commerically, the details of the spray gun or power generator for use with the spray gun will not be described in detail in this specification. However, details of a suitable electrostatic spray device may be found in U.S. Pat. No. 3,645,447 to Cowen and details of a suitable electrogasdynamic generator for such a suitable electrostatic spray device are recited in U.S. Pat. No. 3,651,354 to Cowen. The disclosures of these patents relative to an electrostatic spray device and an electrogasdynamic generator for such a device are incorporated herein by reference in order to avoid an excessively long specification.
While electrostatic spray apparatus is theoretically capable of providing proportional variation in the magnitude of the average depositing field strength of the electrostatic field engendered by the magnitude of the delivery rate of atomizing air for effecting proportional variation in the magnitude of the field strength to provide non-uniform deposition of coating, such critical control of non-uniform spray applications is difficult to attain over large areas of the size of the banded portion of graded windshields and backlights for automobiles. Prior to the present invention, a need existed to take advantage of the higher speed of electrostatic coating to produce a colored band having a desired pattern of intensity free of disturbing optical patterns on a substrate to be laminated into a transparent laminated window having a colored band with a desired pattern of intensity.
2. Description of the Prior Art
U.S. Pat. No. 3,004,875 to Lytle discloses applying a band of metal salt composition at an oblique angle toward an edge portion of a substrate to be coated, using a shield to limit the area of application of the coating composition. The resulting band is of gradually increasing intensity from its boundary with the uncoated portion and the edge of the substrate containing the band. Furthermore, the resulting band has a mottled appearance, particularly in its boundary portion adjacent to the edge of the shield because of eddies that evolve beneath the shield near the edge of the shield during the coating operation.
U.S. Pat. No. 3,113,034 to Fix discloses applying a band of coloring material having a repetitive pattern of curved cut-off line to a continuous sheet of plastic interlayer material using a roller coater to which dye is imparted for imparting said dye to a surface of the sheet while the roller coater simultaneously rotates and reciprocates axially against said sheet while the latter moves past a roll coating station. While this principle of transferring dye has been established, the process has not completely eliminated some residual blur in the applied dye pattern so that the cut-off line between dyed and undyed portions appears fuzzy.
U.S. Pat. No. 3,078,693 to Lytle discloses applying a metal salt composition to a portion of a hot glass sheet to form a metal oxide coating and then laminating the coated glass sheet to one side of a plastic interlayer. The metal oxide films have a mottled appearance which is not removed by lamination.
U.S. Pat. No. 2,088,542 to Westin impinges one or more currents of gas onto an atomized spray of coating composition at a point closer to the substrate to be coated then to the point of origin of the jet to delimit the coated area from the uncoated area. A sharp line of demarcation results between the coated area and the uncoated area.
U.S. Pat. No. 2,082,182 to Schacht uses a shield to confine the flow of pulverulent particles toward a substrate and forms a sharp line of demarcation between the coated and uncoated portions of the substrate.
U.S. Pat. No. 1,861,475 to Hopkins and Odgen uses compressed air to confine the application of a spray of lacquer. The sprayed portion is sharply separated from the unsprayed portion.
U.S. Pat. No. 2,283,253 to Haven controls the width of a line of metal formed by spraying metal particles upon a glass substrate at an oblique angle thereto by providing a guard member positioned between the spray source and the substrate. The spray, on passing the edge of the guard member, causes the formation of eddies that set up back sprays, which form a metal mist upon the glass beyond the area desired to be coated. Gaseous fluid is provided under pressure beneath the guard member to counteract the formation of the eddies and prevent the metal particles from passing beneath the guard member. The resulting boundary between the coated line of metal and the uncoated portion is sharp and abrupt, not gradual as desired for purposes of the present invention.
None of the aforesaid patents relate to applying a colored coating having a main relatively wide portion of essentially uniform intensity and a relatively narrow boundary portion of graded intensity by electrostatic spraying. None of the aforesaid patents was confronted with the problem of having to alter a selected portion of a uniform spray pattern resulting from the mutual repulsion of electrostatically charged particles into a portion of graded intensity. None of the aforesaid patents relate to applying a colored coating of graded intensity to a transparent sheet of relatively rigid material that is one component of a transparent laminated window, assembling the sheet and a clear layer of interlayer material and transferring the coating to the layer of interlayer material while laminating said sheet to said layer at high pressure and temperature conditions regardless of whether the coating is initially supplied in an electrostatic field or otherwise.